French director Anne Fontaine wasn’t familiar with the English term for mature women who prefer much younger men – nor was she aware of the Saturday Night Live sketch, “Motherlover” – but with Two Mothers, she’s melting the snow at the Sundance Film Festival with a love story – “Not a sex story,” she says – about two Australian best friends who fall hard for each other’s teenage sons and form an unconventional quartet.

Close neighbors in an idyllic beach paradise, Lil (Naomi Watts) and Roz (Robin Wright) have been best pals since they were girls. They do everything together – making Roz’s husband (The Dark Knight’s Ben Mendelsohn) feel like an interloper in his own home – and their sons are practically brothers. In the final summer before the boys head off to college, the unspeakable happens when Lil’s son Ian (Xavier Samuel) kisses Roz and she doesn’t stop him from going further. When her son, Tom (James Frecheville), catches them together, he feels compelled to act on his own hormones with Lil.

“I had never read anything like it,” says Watts. “And I loved how I went from a place of quickly judging them to almost instantly forgiving them, and more than that, willing it to happen and to continue. And the question comes up later, and the ‘Oh my God and we have to end this.’ But it’s too good, and that just felt very human to me.”

Fontaine fell in love with the story, which is based on the 2003 novella The Grandmothers, written by British author Doris Lessing when she was 85. “I thought it was powerful that there was no answer at the end,” Fontaine says. “Each one can think what they want to think. No morality.”

Audiences have surprisingly found humor in the movie – it’s clearly not intended as a comedy and treats the romantic relationships very naturally and matter of fact. “The laughter is a way to express a surprise and also something that you are uncomfortable with,” says Fontaine, who took inspiration from Ingmar Bergman’s Persona, which also featured two female characters who became almost interchangeable. “I think in France it will be different.”

In Utah, however, Two Mothers was one of the films cited by the conservative Sutherland Institute in its effort to curtail public funding for Sundance because the festival showcases movies that, in Sutherland’s opinion, fail to reflect the state’s traditional values. But Fontaine believes the movie – though shocking and titillating – strikes a raw nerve only with those who are uncomfortable with the older woman, younger man dynamic, a real-life gender double-standard that is also commonplace on the big screen.

But is there more than one double-standard at work here? What if Two Mothers was instead Two Fathers – with two middle-aged male friends having affairs with each other’s teenaged daughters? “The shock would be less, less, less – depending on how it is done,” says Fontaine. “We see it in real life all the time, men with girlfriends 20 years younger. [Look at] American Beauty, for example, when Kevin Spacey falls in love with the best friend of his daughter.”

Granted, the comedy Blame it on Rio really happened, but Watts was more circumspect: “I think it would be as shocking, but the audience might be less forgiving [of the older men].”